1. Field of the Invention
The present invention relates to a method for manufacturing a liquid jet recording head which performs recording on a recording medium with droplets of recording liquid ejected from a fine discharge opening.
2. Description of the Related Art
A liquid jet recording head such as a thermal ink jet printing head, etc., employed in a liquid jet recording device comprises a plurality of fine nozzles (discharge openings) which eject recording liquid such as ink, etc., liquid chambers each of which is connected to one of the nozzles, and discharge energy generating elements (for example, a heater such as an electrothermal conversion element) each of which is placed in one of the nozzles. Recording is performed by applying driving signals corresponding to the information to be recorded to the discharge energy generating elements, supplying the discharge energy to the recording liquid inside the nozzle in which the discharge energy generating element is placed, and discharging flying droplets of the recording liquid from the fine nozzle.
There are various types of nozzles suggested for use in this type of liquid jet recording head, and one example is explained with reference to FIG. 7.
In FIG. 7, reference numeral 101 denotes a top plate (nozzle member) formed by a silicon wafer which is cut and polished so that the upper surface thereof comprises the <110> crystal plane. The top plate 101 is provided with a liquid chamber 102 which is a hole formed through the top plate 101 and serves to retain the recording liquid therein, and a plurality of nozzle grooves 103 (hereinafter referred to simply as “nozzles”), connected to the liquid chamber 102, for discharging the recording liquid. An element substrate 108 (a heater board 108) comprises a silicon chip in which a number of heating members (heaters) 109 are provided as the discharge energy generating elements.
As shown in FIG. 7, the top plate 101 and the heater board 108 are closely jointed or bonded so that each of the nozzles 103 is arranged to oppose the respective heater 109. The nozzles 103 and the surface of the heater board 108 constitute thin and long discharge nozzles. At this time, the positions of the top plate 101 and the heater board 108 are precisely adjusted to ensure that each of the heaters 109 is placed inside the respective nozzle 103. The recording liquid is supplied from a recording liquid tank (not shown) to the liquid chamber 102 and reaches the nozzles 103. The heaters 109 on the heater board 108 are controlled by a controlling circuit (also not shown) and are individually energized according to printing data. The controlling circuit may be placed on the heater board 108 or may be formed on another substrate.
Each of the heaters 109 individually energized according to the printing data emits heat so as to heat the recording liquid contained in the nozzle 103. The heated recording liquid boils when a crucial temperature is reached and generates bubbles. These bubbles grow in a short period of time, i.e., in several μs, and provide an impact force to the recording liquid. Part of the recording liquid is pushed out from the discharge opening of the nozzle 103 as flying droplets due to the significant force of this impact and reaches the recording medium such as a sheet of paper, etc. An image is printed by repeating these steps.
Next, a method for manufacturing the top plate (nozzle member) 101 will be explained with reference to FIGS. 8A to 8H and FIGS. 8A′ to 8H′, according to steps thereof. It should be noted here that FIGS. 8A to 8H on the left side are sectional views taken along a plane along the liquid discharging direction and FIGS. 8A′ to 8H′ on the right side are end views viewed from the lower side (the surface provided with nozzles) of the top plate.
In FIGS. 8A and 8A′, a silicon wafer 105 which constitutes the top plate (nozzle member) provided with the liquid chamber and the nozzles, has a <110> crystal orientation at the surface and a <111> crystal orientation in the longitudinal direction of the nozzles. A silicon dioxide (SiO2) thin-film 106 of 1 μm in thickness is formed on both sides of the silicon wafer 105 by a deposition process such as a thermal oxidation process or a chemical vapor deposition (CVD) process, as shown in FIGS. 8B and 8B′ The silicon dioxide thin-film 106 functions as a mask layer during anisotropic etching of the silicon. Then, one surface (the surface which will be provided with nozzles, hereinafter referred to as the “nozzle surface”) of the silicon dioxide thin-film 106 is patterned into a shape of the nozzles and the liquid chamber combined, and the other surface is patterned into a shape of the liquid chamber by using a standard photolithography technique (FIGS. 8C and 8C′). The nozzle surface is coated with a silicon nitride (SiN) layer 107 by a method such as a CVD method (FIGS. 8D and 8D′) and is patterned into the shape of the liquid chamber (FIGS. 8E and 8E′).
Then, anisotropic etching is performed by immersing the wafer in an etchant such as a 22% tetramethylammonium hydroxide (TMAH) solution. Etching progresses along the exposed portion (i.e., the portion having the shape of the liquid chamber) of the silicon from both sides of the wafer during the anisotropic etching step, resulting in the formation of a through hole (the liquid chamber 102) as etching progresses (FIGS. 8F and 8F′).
The shape of etching shown in FIGS. 8F and 8F′ will be described below. The main purpose of the anisotropic etching is to form fine nozzles in the top plate (nozzle member) 101. Accordingly, patterning of the nozzles is selectively performed so that the <111> plane of the silicon is parallel to the nozzle wall. When the liquid chamber 102 is substantially rectangular, a shorter side of the liquid chamber (through hole) 102 is left with a perpendicular plane after the etching because the <111> plane is provided perpendicular to the surface of the wafer. In contrast, since a longer side of the liquid chamber (through hole) 102 comprises a number of <111> planes tilted by approximately 30 degrees relative to the wafer surface, the surface of the longer side, composed of a number of planes, is not as perpendicular or smooth as that of the shorter side in a strict sense.
Next, the silicon nitride layer 107 is removed by etching (FIGS. 8G and 8G′) to expose the nozzle pattern formed in the silicon dioxide thin-film 106 of FIGS. 8C and 8C′. Anisotropic etching using the TMAH solution is performed again so as to etch the portion corresponding to the nozzles (FIGS. 8C and 8C′).
Because the <111> plane perpendicular to the wafer surface is provided in the liquid discharging direction, the cross-section of the nozzles 103 obtained by anisotropic etching is rectangular. In contrast, because there is no surface to inhibit the etching in the longitudinal direction of the nozzle, a nozzle wall 104 (see FIG. 9A) provided between the nozzles is etched from the rear side (the liquid chamber side) as well as the front side of the nozzle, resulting in over-etching in the longitudinal direction, forming an angular shape. Accordingly, the silicon dioxide thin-film serving as a mask layer may remain on the over-etched portion. In order to remove the silicon dioxide thin-film, high-pressure air or high-pressure air containing water is sprayed on the wafer to remove only the silicon dioxide thin-film without damaging the silicon. A pressure of 100 to 200 kPa is sufficient for removing the thin-film of approximately 1 μm in thickness when the method of spraying water by high-pressure air is performed. The entire silicon dioxide thin-film may also be removed by wet etching using a liquid mixture of ammonium fluoride and hydrofluoric acid.
The shape of the top plate (nozzle member) 101 fabricated by the above-described process is shown in FIGS. 9A to 9C. When the patterning is performed to form the liquid chamber, both sides of silicon are patterned substantially the same in the drawings; however, the size of the pattern at the recording liquid supplying side (i.e., the upper surface in FIG. 7) may be reduced as long as a penetrating hole can be formed by anisotropic etching. From the point of view of connection with a recording liquid supplying member (not shown) and securing the wafer strength during the formation of the top plate, it is preferable that the pattern be smaller than that on the nozzle side.
Because the above-described conventional method for fabricating the top plate (nozzle member) employs a silicon anisotropic etching technique to fabricate the top plate, the top plate can be produced by wafer-scale fabrication techniques, enhancing mass-productivity. Also, since a photolithography technique is employed to form the nozzles, the nozzles can be precisely formed with high density. However, the shape of the liquid chamber formed by anisotropic etching is complex, as shown in FIGS. 9A to 9C. More particularly, whereas the <111> plane of silicon is perpendicular to the surface of the side wall in the longitudinal direction of the nozzle, there is no independent <111> plane in the arraying direction of the nozzles and the <111> plane meets the wall surface at an angle of 55 degrees and at an angle of 71 degrees in the nozzle arraying direction. Consequently, when anisotropic etching is performed to form the liquid chamber, the liquid chamber is over-etched in the nozzle direction, leaving these two surfaces at the corners, and the resulting liquid chamber 102 has the complex shape shown in FIGS. 9A to 9C. The liquid chamber is etched for a time period sufficient to form a penetrating hole. Since a typical wafer is approximately 0.6 mm thick from the point of view of strength, the wafer is subjected to anisotropic etching of approximately 0.3 mm in depth when the top plate is fabricated according to the steps shown in FIGS. 8A to 8H and 8A′ to 8H′. Since the over-etched amount in the nozzle direction is substantially the same as the depth, the chip size must be undesirably large, resulting in an inefficient chip size, and there is a problem in that the number of chips fabricated from one wafer is significantly reduced. Also, because the angled surfaces relative to the nozzle arraying direction remain in the vicinity of the liquid chamber inner side surfaces, flow resistance of the recording liquid differs according to the difference in the shapes of the liquid chamber at both ends and at a center region. In other words, because the conditions for refilling the recording liquid (the nozzles are refilled with recording liquid after being discharged) vary, discharge characteristics vary among the nozzles, causing printing quality to vary and performance of the head to degrade.